This study examined the internal and external validity of the Japanese version of the Thinking Styles Inventory (TSI: Hiruma, 2000), which was originally developed by Sternberg and Wagner (1991) based on the framework of Sternberg’s (1988) theory of mental self-government. The term “thinking style” refers to the concept that individuals differ in how they organize, direct, and manage their own thinking activities. We administered the Japanese version of the TSI to Japanese participants (N = 655: Age range 20—84 years). The results of item analysis, reliability analysis, and factor analysis were consistent with the general ideas of the theory. In addition, there were significant relationships between certain thinking styles and 3 participant characteristics: age, gender, and working arrangement. Furthermore, some thinking styles were positively correlated with social skill. Implications of these results for the nature of Japanese thinking styles are discussed.